It is known to polymerize solutions comprising alkenyl aromatic monomers having a diene rubber dissolved therein to form polyblends having a matrix phase of polymers of said monomers having dispersed therein particles of said diene rubber grafted with said monomers.
Mass and mass/suspension processes have been used to prepare such polyblends. U.S. Pat. No. 3,903,202 is one such suitable process for the continuous mass polymerization of such polyblends and it is hereby incorporated by reference.
The morphology of the rubber particles dispersed in the polyblend is critical to the final properties of the polyblend. Generally, the larger the size of said rubber particles, the greater the toughness and the smaller the size, the higher the gloss. Hence, the size of the rubber particles must be controlled to insure the control of the properties of the polyblend. U.S. Pat. No. 3,903,202 discloses that agitation during the early phases of polymerization disperse the dissolved rubber as particles and that higher rates of agitation generally decreases the size of said particles with lower rates of agitation producing larger particles. Controlling agitation rates then provides a means for sizing said rubber particles. It has been found that lower rates of agitation, however, creates problems of heat transfer and inhomogeneity that are difficult to control when operating at the lower agitation rates needed to produce larger particles. This is particularly true in continuous processes wherein large amounts of monomers are being polymerized and heat and homogeneity control are imperative to process and product control.
The art has disclosed other methods for sizing the rubber particles other than by agitation rates. U.S. Pat. No. 3,660,535 discloses a method wherein a partially polymerized solution of rubber and monomers is recycled into a stratifying type polymerizing system having linear flow. The effluent from the first reactor, having a conversion of about 20 to 38%, is recycled as a partially polymerized solution in varying amounts to the feed stream at a location of essentially zero conversion and prior to phase inversion in the first reactor.
This procedure has been found to vary the particle size of the rubber during the inversion of the rubber phase, however, control has been found to be difficult because the recycle stream contains grafted rubber particles that have varying levels of graft varying with levels of conversion in the recycle stream. The grafted rubber particles in the rubber phase has been found to act as a soap or dispersing agent for the rubber particles in the monomer-polymer phase during inversion of the two phases, hence, minor variations in graft or conversion have a profound effect on particle size control.
This is particularly evident as polymerization is carried out in a linear flow stratifying reactor system wherein inversion gradually occurs as the feed stream is progressively polymerized from 0% to 20 to 38% conversion down through the stratified reactor.
U.S. Pat. No. 3,398,214 disclosed a batch mass polymerization process wherein a partially polymerized monomer batch of 30-40 conversion is blended with a partially polymerized rubber-monomer batch of less than 10% conversion and the blend is then completely polymerized by batch polymerization. The process is operable only when the monomer-rubber solution is prepolymerized to not greater than 10% conversion which provides very small rubber particles for high gloss in molded parts. High toughness is only developed when the monomer-rubber solution is polymerized to about 3% or less. Such processes do not provide high efficiency at such low conversions.
U.S. Pat. No. 3,488,743 discloses a process wherein a solution of monomer, low molecular weight rubbers and polymer are batch mass polymerized then finished by batch suspension polymerization.
Such batch process produce polymers having relatively small rubber particles which do not efficiently toughen the polyblends having relatively low elongation at fail. Beyond properties, the batch processes require long conversion cycles to graft, invert and disperse the rubber phase, hence, having high energy requirements.
The sequence of steps is critical in the process. The first reaction zone is operating under steady state conversion of about 15 to 50% conversion, such that as said first monomer-rubber solution enters said first reaction zone the dissolved rubber inverts rapidly into dispersed rubber particles. The first reaction zone is operating under back mixed steady state polymerization such that the partially polymerized solution is homogeneous as to temperature, viscosity and chemical composition causing said dissolved rubber to invert and be sized into first rubber particles or a smaller size in a continuous and controlled operation.
The present invention provides that the second monomer-rubber solution fed simultaneously, has present a sufficient amount of a polymer of said monomers such that said dissolved rubber inverts and is dispersed rapidly into second rubber particles, said second rubber particles having higher levels of said polymer of said monomer as occluded polymer during inversion, hence, producing larger rubber particles. During the inversion of the dissolved rubber, there is essentially no grafted rubber particles in the rubber phase as a soap to decrease the particle size, hence, larger particles can be formed at the higher agitation rates needed for heat transfer. Here, the larger the concentration of the polymer added to the second monomer-rubber solution, the larger the second rubber particle, providing a means for readily controlling the rubber particle size. As the first and second rubber particles move through the first and second reaction zones the rubber particles develop a grafted rubber phase which stabilizes the rubber particles at the small and large sizes formed during inversion so that particle size is controlled giving a final product with a bimodal particle size distribution.
A preferred mass polymerization process for preparing polyblends with a wide range of particle sizes ranging from 0.5 to 10 microns has been disclosed in copending application Ser. No. 18,388 filed of even date herewith in the name of Raymond D. Burk and is hereby incorporated by reference. The process of the copending application is disclosed as:
An improved process for the continuous mass polymerization of a solution comprising a alkenyl aromatic monomer having a diene rubber dissolved therein comprising the steps:
A. continuously charging said solution of an alkenyl aromatic monomer having a diene rubber dissolved therein to a first flow through reaction zone,
B. continuously polymerizing said solution under back mixed agitation and steady state conditions, said monomer being polymerized to an average conversion of about 15 to 50%, said diene rubber being dispersed as diene rubber particles having present grafted and occluded polymers of said monomer in amounts of about 1 to 5 parts per 100 part of said diene rubber, said solution becoming a partially polymerized solution,
C. continuously withdrawing said partially polymerized solution from said first reaction zone,
D. continuously charging said partially polymerized solution to a second flow through reaction zone and further polymerizing said partially polymerized solution to about 20 to 95% conversion under substantially linear flow,
E. continuously removing an effluent from said second reaction zone and continuously separating a polyblend from said effluent, said polyblend having a matrix phase comprising said polymerized monomer having dispersed therein said diene rubber particles,
F. the improvement comprising: said solution being continuously charged in step (A) having present a polymer comprising said monomer in amount sufficient to control the size of said rubber particles being dispersed in step (B).
It has been found that polyblends having a dispersed rubber phase can be improved in toughness and gloss if the rubber particles have a bimodal particle size distribution wherein about 50 to 95% by weight of the particles have a weight average particle size diameter of about 0.5 to 1.5 microns as first rubber particles and 5 to 50% by weight are 2 to 10 microns, preferable 2 to 5 microns as second rubber particles.
Such polyblends have been prepared by melt blending two polyblends having small and large particles, however, this requires batch operations and additional energy requirements to melt blend the two polyblends.
U.S. Pat. No. 4,012,462 discloses a batch mass/suspension polymerization process for preparing polyblends with high toughness having a polydisperse or broad particle size distribution of relatively large particles, however, such polyblends are deficient in gloss. Canadian Pat. No. 832,523 has disclosed polyblend compositions prepared with a bimodal rubber particle size distribution of relatively large particles, however, such polyblends have relatively low gloss.
U.S. Pat. No. 3,652,721 discloses a method for preparing ABS polyblends having a bimodal rubber particle size distribution wherein emulsion polymerized and grafted particles of less than 0.25 microns are blended with agglomerated emulsion polymerized and grafted rubber particles of about 0.3 to 1.0 micron. Such polyblends have high gloss and toughness but require large percentages of relatively small rubber particles in the polyblend to reach the desired toughness for engineering plastics, i.e., about 25% rubber.
It is the objective of the present invention to provide a continuous mass polymerization process for preparing polyblends having about 2 to 15% of a dispersed rubber phase with a bimodal rubber particle size distribution, formed in situ during polymerization, providing a more simple and easily controlled process with lower energy requirements.